With rapidly advancing computer, network, and digital imaging technologies, there is an astronomical amount of video data for a wide range of applications, especially in digital playback devices such as digital video recorders, high-definition televisions, home theater computers, video conferencing devices, smart phones, and so forth. Thanks to the readily availability of consumer-grade and production-grade 360-degree cameras as well as the promotion of 360-degree video platforms by social media giants such as YouTube and Facebook, the fast adoption of 360-degree videos has been boosted throughout the years. It is expected that 360 videos will become a dominant video playback format in the near future.
360-degree videos give a viewer an immersive experience through displaying full surrounds of a camera in a spherical canvas. In many circumstances a viewer has a limited field of visual attention, and studying how to display 360-degree videos during playback has thus emerged as an increasingly important problem. Hand manipulation and virtual reality are so far two primary ways for displaying 360-degree videos on a device with a natural field of view (NFoV). In hand manipulation, a viewer navigates a 360-degree video via a sequence of mouse clicks, and whereas in VR, a viewer uses embedded motion sensors in a VR headset for navigation. Note that both HM and VR would require a viewer to select a viewing angle for each frame, while the FoV is defined by the device. However, such selection mechanism could be cumbersome and tedious, particularly for video footage in which subjects are actively moving and changing their locations continuously. In fact, recent research studies have shown that both HM and VR could cause a viewer to feel fatigue and discomfort.